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The spacefaring human civilization in Star Trek: The Next Generation is way ahead of us, but even they had nothing on the immense Dyson sphere the Enterprise encountered.
Dyson spheres are the stuff of sci-fi for now, but maybe not in the future. A (hypothetical) alien civilization advanced enough to harvest energy from the star they orbit may be able to build a structure, or swarm of structures that can harvest its energy. But would the same thing work if they tried it with a black hole? Astronomer Tiger Yu-Yang Hsiao, of National Tsing Hua University in Taiwan, led a team of researchers who investigated whether they could.
Such a civilization would need an energy source greater than most stars. Hsiao and his colleagues, whose study was recently published in Monthly Notices of the Royal Astronomical Society, believe there are several energy sources in a black hole that could be the ultimate source of power for intelligent aliens—so long as they don’t get too close.
“The accretion disc can emit 104-109 times more energy than the Sun,” Hsiao told SYFY WIRE on behalf of his team. “Additionally, if aliens can collect all the energy from the jets as well, they will have five times more total energy. We think these two, accretion disc and jets, yield the most energies that civilizations can harvest.”
But first, that civilization would have to be at least on par with Star Trek. They would have to keep up with the Kardashev scale. The brainchild of Russian astronomer Nikolai Kardashev, which first appeared in his 1964 paper Transmission of Information by Extraterrestrial Civilizations, originally defined three main types of civilizations. We are barely a Type 1, which is supposed to be able to harness all the energy on its planet through nuclear fusion, antimatter and renewable energy. Humans are on their way there—about a 0.7 according to Carl Sagan.
A Dyson sphere would need at least a Type II civilization to operate it. Beings in a Type II civilization are thought to have already figured out how to put all of their star’s power to use, and would be able to collect that energy through a megastructure known as a Dyson sphere. Theorized by astronomer and physicist Freeman Dyson in 1960, it would be a gargantuan shell or ring (as he imagined it) of energy harvesters that would surround a star to collect its energy and power the entire civilization. This thing could be close in size to Earth’s orbit.
So many questions. Remember that this is the idea of a Dyson sphere around a star. Black holes can be anywhere from several solar masses to millions, which is why Hsiao and his team modeled scenarios around models of black holes that were 5, 20 and 4 million times the Sun’s mass (that 4 million is the mass of our galaxy’s supermassive black hole, Sagittarius A*). If aliens did manage this with a black hole, which of its processes would give them the most efficient methods of collecting energy?
“The accretion disc plus jets would be most efficient,” said Hsiao. “To make it more efficient, the key is thermal endurance. If the Dyson sphere can survive higher temperature, it can be smaller and closer to the black hole. It is also possible that the total energy of a jet, including its kinetic energy, can be transformed into other forms of energy like electricity as in a power plant.”
If Dyson spheres are really out there, we would have to actually observe one for proof. That could be complicated. Excess radiation from a Dyson sphere could make the accretion disc of a black hole glow brighter, but black holes that are actively feeding also intensify in brightness, so how to distinguish between that process and an alien-built structure would have to be figured out somehow. Hsiao does believe in the possibility of a Dyson sphere existing around a black hole whose accretion disc shows up as especially energetic. He is, however, realistic about it.
“Performing model fitting would help us reveal such an artificial structure,” he said. “However, strong evidence is required to convince scientists and the public. For example, radial velocity and the light curves would help to identify possible origins.”